The concept of implementing a phased-locked loop (PLL) technique in a digital signal processor (DSP) for demodulating a received FDMA signal is fairly known in the art (see, for example, Jacob Klapper and John T. Frankle, Phase-Locked and Frequency Feedback Systems, Academic Press, New York, 1972; Ch. 8). However, these techniques were employed usually for carrier recovery rather than for processing the received message and envelope demodulation.
A method utilizing a PLL technique for coherent detection and demodulation of a FDMA signal, that is a superposition of amplitude modulated carriers is described in U.S. application Ser. No. 09/575,517. An array of special digital phase-locked loops (PLLs) is implemented in a DSP for processing the received signal after it is digitized, and for extracting the envelopes corresponding to each carrier. The quality of the envelope estimate depends on the ability of the DSP to overcome all sources of inaccuracies such as coupling between the individual carriers and harmonic distortions while preserving a spectral bandwidth covering the spectrum of the modulating signal. In addition, the demodulation process should be performed in real time and introduce only a short processing delay.
It should be noted that one of the most time consuming operations when using a PLL technique for envelope extraction is associated with generation of sine and cosine waveforms. One common approach for the synthesis of the trigonometric functions is to build a lookup table where the exact values of the sine and/or cosine functions are stored up to a desired accuracy. This method was further enhanced by interpolation between table entries (see, for example, U.S. Pat. No 4,905,177).
Alternatively, sine and cosine waveforms may be synthesized by using a real-time solution of a difference equation (see, for example, U.S. Pat. No 4,888,719). Several enhancements to these methods were disclosed (see, for example, U.S. Pat. Nos. 5,113,361, 4,761,751, 5,631,586). Despite the apparent superiority of the difference equation method, due to the finite precision of the computer, implementation of difference equation solution in DSP may produce an accumulating error. The error may lead to both phase and amplitude drift (see, for example, U.S. Pat. No 4,285,044) affecting the accuracy of the envelope calculation, and thus a control mechanism is required.
Synthesis of trigonometric functions by employing the prior art technique is a complicated task, requiring relatively high computational load and large storage space for storing the computed data, since such synthesis is performed for a digital sampled signal divided into frames consequently frame by frame for each sample of the frame. This may adversely effect the PLL performance and interfere with real-time and memory requirements.
There is, accordingly, a need in the art to provide an improved technique that substantially reduces the drawbacks of the hitherto known techniques for generation of the trigonometric functions within a PLL in general, and, in particular, when the PLL is utilized for amplitude demodulation and envelope extraction in particular for real-time applications.